Production of nitroaromatic compounds from the mixed acid nitration of benzene or toluene is accompanied by the production of nitrophenolic by-products. It is common industrial practice to remove these nitrophenolic by-products from the nitroaromatic hydrocarbon reaction product by an aqueous alkaline washing step. This alkaline aqueous waste stream containing the nitrophenolic by-products is also saturated with nitroaromatic hydrocarbons.
Reduction of the concentration of priority pollutant nitrophenolics and nitroaromatic hydrocarbons to less than one part per million (sub-ppm) in aqueous waste streams discharged to the environment is a very recent requirement in most industrialized nations. The specific priority pollutants addressed are 2-nitrophenol, 4-nitrophenol, 4,6-dinitro-o-cresol, 2,4-dinitrophenol, 2,6-dinitrotoluene, 2,4-dinitrotoluene and nitrobenzene.
U.S. Pat. No. 4,604,214 discloses a process for removing trinitrocresols and picric acid contaminants from a wastewater stream generated in the production of nitroaromatics. The process involves contacting the crude dinitrotoluene generated by the mixed acid technique with an alkaline medium to generate an alkaline wash water containing water soluble nitrocresols and picric acid. The wash water is treated with aqueous acid in sufficient amounts to reduce the pH to 3-4. After pH adjustment the aqueous medium is contacted with hydrogen peroxide and a ferrous ion under conditions to effect oxidation of a substantial portion of the trinitrocresol to carboxylic acid, nitric acid and carbon dioxide.
U.S. Pat. No. 4,804,480 discloses a process for destroying polynitrophenols or their salts in an aqueous waste by treating with at least 2 moles of hydrogen peroxide per mole of polynitrophenol in the presence of from 0.002 to 0.7 moles of an iron salt per mole of polynitrophenol. The destruction takes place at a pH lower than 4 and a temperature greater than 65.degree. C.
U.S. Pat. No. 4,925,565 discloses a process for the reprocessing of nitrophenolic by-products, which are contained in the wastewater from a nitration, through solvent extraction, distillative recovery of the solvent and recovery of a nitrophenolic residue and incineration of the nitrophenolic residue.
Incineration or wet air oxidation are processes which are not economically viable for aqueous streams that are relatively low in organic content (about 1 wt %). Most other treatment methods involve chemical reaction, e.g. ozonolysis or Fenton's reagent (hydrogen peroxide and ferrous ion). The chemical reaction rate drops precipitously as the concentration of the priority pollutant begins to approach the low levels required. In order to achieve the low, allowable concentrations of priority pollutants, large excesses of the chemical reactant must be added.
For example, the oxidative destruction of the bulk of the nitrophenols with Fenton's reagent is cost effective when the target range is 10-100 ppm. The oxidation of the nitroaromatic hydrocarbons is, however, much less efficient. Efforts to achieve sub-ppm concentrations on all priority pollutant nitrobodies present using Fenton's reagent alone requires so much hydrogen peroxide that other stand-alone treatments such as carbon treatment are more cost effective.
Thus, carbon adsorption is the most obvious treatment technology; however, as a stand alone technology it suffers certain drawbacks. One such drawback is the volume of carbon required to treat an industrial alkaline waste stream containing both nitroaromatics and nitrophenols. Carbon treatment alone allows for a certain loading of the nitrobody species to be adsorbed on the carbon bed before the nitrophenols are displaced from the carbon by the nitroaromatic hydrocarbons (the chromatographic effect), i.e. the nitrophenolics are less strongly adsorbed.
Since the regulated nitroaromatic hydrocarbons displace the nitrophenols, only four of which are presently regulated, from the carbon bed, an obvious approach is to remove the nitroaromatic hydrocarbons from the alkaline waste stream before passing it through the carbon bed. Such pretreatment removal of the nitroaromatics would involve biodegradation of the waste stream with suitable microorganisms or precipitation of the nitroaromatic hydrocarbon by chilling the waste stream.